An electric utility typically purchases power from numerous power plants. These power plants are all connected to a power grid maintained and operated by the electric utility. The power plant makes electricity and delivers it to the grid.
Between a power plant and an electric utility lies a “point of interconnection.” This is the point at which electrical power actually changes hands, from being under control of the power plant to being in control of the grid. Thus, by the time power reaches this point of interconnection, it is expected to be ready for distribution. This means the utility expects the delivered power to have certain electrical characteristics that render it suitable for distribution.
In addition, the electric utility expects the power plant to assist in supporting the grid in times of stress. For example, there may be electrical disturbances that require a rapid injection of reactive power or additional voltage to stabilize the voltage on the grid. The electric utility expects the power plant to be ready, willing, and able to provide reactive power or voltage support during such a disturbance. A power plant capable of functioning in this way is said to be “grid friendly.”
Because of their years of experience with rotating machines, such as those used in conventional thermal power plants and hydroelectric power plants, electric power utilities have come to expect power plants to behave as if their power were generated using a conventional rotating machine. This expectation imposes a burden on purveyors of electricity produced by unconventional devices, such as fuel cells, photovoltaic cells, and wind turbines. Power produced by such devices does not always behave as if it were produced by a rotating machine. For example, photovoltaic cells naturally produce DC, whereas rotating machines naturally produce AC.
Requirements for grid connection have grown even more stringent over the years. In North America, the current trend is toward more rigid standards for wind and solar power supplies. An example of the evolving standards is the “Interconnection Standards Review Initiative, Draft Straw Proposal” as set forth by CAISO (California Independent System Operator) in the spring of 2010.
As a result of such standards, there is a growing need to provide ways for unconventional power sources to more closely match the electrical characteristics of rotating machines in order to participate in power distribution on a utility grid.